Joty— Useful Methods of Teaching Elementary Physics. 219 
occurs from 7, to 72, a given mass of gas will change in volume 
from V, to V2, so that 
VY, iT, 
Ve” i 
We also explain that we may express 7 as 273 = ¢°, where ¢° is 
temperature read on an ordinary centigrade thermometer. With 
these data we now can combine the law of Boyle with the law of 
the expansion of a gas, in the usual manner, arriving at the result 
ae 2 he VaP. } 
er « Briea ee 
Cooling of a gas upon expansion. Presence of moisture in the 
atmosphere. Snow on high mountains.—A striking experiment, 
teaching a great deal, is the following:—A strong copper vessel 
(preferably spherical, about 6 cm. diameter) is fitted with a 
screw valve and attachment, so that it can be connected with a 
bottle of liquid carbon dioxide, and some of the liquid transferred 
to it. This transference is facilitated by cooling the copper 
vessel with a little ether when the connexion is made. 
The copper vessel, clean and bright upon the outside, is hung 
by a wire from a retort-stand ring, and the valve screwed open 
before the class. In a few moments a thick covering of white 
frost overlays the bright copper, and continues to grow into a. 
thick shell of ice. Finally this melts and drops off the sphere as 
water. Let the student, by tasting, see that this is water, and not 
something derived from the escaping gas. The experiment may 
be repeated, placing the vessel in a small beaker containing a 
little phosphorus pentoxide or calcium chloride, and letting the 
gas escape while the beaker is closed by a cardboard cover. No 
ice, or very little, now collects. But without the repetition of the 
experiment it is quite apparent that :— 
1. The gas leaving the sphere has permitted the residual gas 
to expand, and this has produced a very marked cooling of the 
vessel. . 
2. The air must contain a great deal of moisture to account 
for all the ice upon the vessel. 
3. The air gives up its moisture when cooled. 
